A lithium ion secondary battery is widely used as a power source for, e.g., laptop computers, video cameras, mobile phones, electric vehicles, and hybrid automobiles. The structure of a lithium ion secondary battery is outlined as follows. A lithium ion secondary battery is mainly constituted by, e.g., a packaging material (made of, e.g., aluminum, stainless, etc.) for a case constituting a battery main body, a tab lead (made of, e.g., aluminum, nickel, nickel plated copper, etc.) configured to extract electrons from a positive electrode, a negative electrode, and a pole plate, and a separator. As an electrolyte filled in a battery, a non-aqueous fluorine compound such as LiPF6 is used, and charging and discharging are carried out by the lithium ions transmitting electrons between both poles.
Among members constituting the lithium ion secondary battery, a packaging material for a case and a tab lead are subjected to a surface treatment for the purpose of rust prevention for the members and improvement of adhesion of the laminated film and then laminate coated.
The corrosive environment inside a lithium ion secondary battery will be explained. As explained above, in a lithium ion secondary battery, a non-aqueous fluorine compound solute such as LiPF6 and a non-aqueous solvent such as ethylene carbonate (EC) and dimethyl carbonate (DME) are used as an electrolyte. The fluorine compound hydrolyzes from deterioration with aging inside the battery and from a small amount of moisture mixed in the battery during the production process, and highly corrosive hydrofluoric acid (hereinafter may be referred to as “HF”) is formed. A very small portion of the HF and the non-aqueous solvent penetrate inside the laminated film and finally reaches an interface of the undercoating. However, if the undercoating does not have sufficient HF-resistance and solvent resistance, problems, such as, e.g., detachment of laminated films due to dissolution of the undercoating, corrosion of members, causing leakage of electrolyte, or deterioration of charging and discharging rate, may occur. These problems are especially problematic in a secondary battery for automobiles in which extremely high safety standards are required. For that reason, it has been strongly desired that developments of an undercoating having high HF-resistance and solvent resistance.
As a laminating undercoating, a chrome-free type film is desired in view of the recent reduction of environmental burdens. In Patent Document 1, an undercoating including effective fluorine ions, zirconium ions, aluminum ions, and polyitaconic acid is disclosed. Also, in Patent Document 2, an undercoating including a basic zirconium compound, and/or a cerium compound, a carboxyl group containing resin, and an oxazoline group containing acrylic resin is disclosed. Both undercoatings have excellent early adhesion and corrosion resistance as a laminating surface treatment. However, in an environment in which HF-resistance and electrolyte resistance under high temperatures assuming the summer are required, there was a problem that the adhesion strength of the laminated film decreases when the film composition, especially the metal composition as a crosslinking agent, elutes due to HF.
Patent Document 3 discloses a laminating surface treatment agent in which a water-soluble polymer and a trivalent chromium compound are mixed at a certain ratio. Furthermore, Patent Document 4 discloses a polymer battery packaging material in which a laminated film is arranged after chromate-phosphate treatment. Trivalent chromium is contained in both of the undercoatings. However, since trivalent chromium is a crosslinking agent excellent in HF-resistance, it is possible to maintain good film adherence even in an environment in which HF exists. However, since it contains chromium, it conflicted with the current trend to reduce environmental burdens.
Patent Documents 5, 6 and 7 disclose a laminating surface treatment agent containing chitosan or its derivative as a main component. Patent Documents 5 and 6 are characterized in that metal crosslinking agents such as Zr, Ti, and Hf are contained. These metal crosslinking agents are extremely effective for accelerating polymerization of chitosan group and can give excellent initial adherence and water resistance to the undercoating. However, in the same manner as the problems in Patent Documents 1 and 2, in the HF and electrolyte, the metal crosslinking agent elutes to destruct the crosslink of the undercoating, which prevents long term maintenance of adherence of the laminated film.
Patent Document 7 relates to a tab lead material characterized in that a tab lead is subjected to a surface treatment with chitosan or chitosan derivative and then an insulating film is mounted. Patent Document 7 is characterized to improve laminated film adherence in a fluorine contained non-aqueous electrolyte. But the durability of adherence of the laminated film in an environment in which a lithium ion secondary battery used for automobiles is exposed was not sufficient, and therefore it was difficult to maintain the adherence for a long time.    Patent Document 1: Japanese Unexamined Laid-open Patent Application Publication No. 2008-297595    Patent Document 2: Japanese Unexamined Laid-open Patent Application Publication No. 2009-84516    Patent Document 3: Japanese Unexamined Laid-open Patent Application Publication No. 2004-79402    Patent Document 4: Japanese Unexamined Laid-open Patent Application Publication No. 2001-202927    Patent Document 5: Japanese Patent No. 4081276    Patent Document 6: Japanese Unexamined Laid-open Patent Application Publication No. 2006-40595    Patent Document 7: Japanese Unexamined Laid-open Patent Application Publication No. 2008-27771